In this episode, we chart the WHO smallpox eradication campaign - an international operation of massive manpower, unlikely allies, and sheer human determination. From the concept of "ring-fence" vaccination beginning with forest firefighters in America, to a junior doctor striking a bargain with smugglers on the India-Bangladesh border, it's a story that spans several continents and reveals how people's unique ability to shape the world can, just occasionally, be used for good.